1. Field of the Invention
The present invention relates to a system for detecting an engine speed in a multi-cylinder internal combustion engine. The present invention is suitably adapted for detecting an engine speed in an internal combustion engine, such as a V-type engine, a horizontally opposed cylinder engine, a W-type engine or a star-type engine, provided with a plurality of sets of camshafts for operating at least the intake valves or exhaust valves of respective cylinders, by detecting the engine speed from the rotation of the camshaft.
2. Description of the Related Art
In the prior system for detecting an engine speed in a multi-cylinder internal combustion engine, for example, shown in Japanese Unexamined Publication No. 55-82965, in order to detect accurate engine speed irrespective of the rotational speed change of the crank shaft due to the combustion, a mean engine speed is calculated from a time in which the crankshaft, rotates for a predetermined crank angle (CA). Such a predetermined crank angle is 720/4 degrees for four cylinder engine or 720/6 for a six cylinder engine, and is selected in order to prevent variation of the obtained data.
However, when using this technique in an internal combustion engine of the overhead camshaft (OHC) type with two or more rows of cylinder blocks, such as a V-type engine or a horizontally opposed cylinder engine, the engine speed must be calculated from the rotational speed of camshafts separately provided for operating intake and/or exhaust valves at each of the cylinder blocks. In these kinds of engines, therefore, a precise detection of the mean engine speed over a tme of 720/6 degrees becomes difficult, because a change of the driving force is attained at a CA cycle equal to 240 degrees (V6 type engine) which is longer than the 720/6 degrees. If the prior method used in the straight-line type engine is applied for detection of the mean speed in the V-type engine, therefore, a time would be measured for allowing one of the camshafts to rotate for a crank angle of 120 degrees. Thus the angle of change of speed of the camshaft, 240 degrees, is longer than the angle for detection of the engine speed, 120 degrees. Therefore, a precise measurement of the mean engine speed becomes impossible, so that there is a large possibility of an error in the obtained result. Thus, precise control of an engine operation, such as ignition timing control or fuel injection control by the obtained mean engine speed becomes difficult.